Segmentation of similar grayscale values

Hi

I need to segment this image data in two phases. The tubular structures are carbon fibres that need distinguishing from the background. I was wondering if anyone could point me in the right direction.

I was thinking of some kind of percolation algorithm that would consider the white and black regions at the interface and according to the closest neighbor would assign it to either white (fibre) or black (background). Any other methods that could distinguish the background from the image would be appreciated. I am unsure on how to proceed or if there are pre made plugins that can execute the task already.

Many thanks

Many thanks.